Method for producing tantalum sheet



1967 c. G. DUNNVXETAL. 3,335,037

METHOD FOR PRODUCING TANTALUM SHEET Filed Dec. 27, 1953 /m ii 00Inventors: Cecilpciig/ugm t Ha old e 8 er bywi Th (air A "Etc rm ey.

United States Patent Oke 3,335,037 METHOD FOR PRODUCING TANTALUM SHEETCecil G. Dunn and Harold'F. Webster, Scotia, N.Y., as-

signors to General Electric Company, a corporation of New York FiledDec. 27, 1963, Ser. No. 334,049 2Claims. (Cl. 14813.1)

This invention relates to a process for producing refractory metalbodies and more particularly to a process for producing tantalum foilshaving a (110) preferred grain orientation.

Tantalum metal has, as a result of increased knowledge of its propertiesand with improved production techniques, been used in widespread anddiverse applications in recent years. It has a high melting point, goodstrength, chemical inertness to many strongly corrosive materials and ithas been used as electrode material in diodes filled with cesium vapor.

The work function of a clean metal crystal, its adsorption properties,and its work function when thinly coated with cesium all are stronglydependent upon the crystallographic orientation of its surface. Thus,when a thermionic diode is built with its cathode and anode ofpolycrystalline metal with random grain orientation, the emissioncurrent density is non-uniform over the surface. Certain grains may emitas much as 100 times the current density emitted by the poorly emittinggrains. When the diode is used to convert heat to electrical energynon-uniformity of the work function of the anode will also degrade thedevice efiiciency. If the electrodes can be made of many grains havingapproximately the same crystallographic orientation, the diodeperformance will be improved by having the entire electrode surfaceoperating at maximum efficiency. The most strongly emitting grains oftantalum in cesium vapor are those having a (110) surface and. anelectrode made up preferentially of grains of this crystal face will bethe optimum cathode.

It is a principal object of this invention to provide a process forproducing tantalum foil having a stable (110) grain orientation.

It is a further object of this invention to provide a process forproducing tantalum foil having improved electrode work functionproperties in cesium filled diodes.

A further object of this invention is to provide a process for producingtantalum foil having the (110) preferred grain orientation.

Other objects and advantages of this invention will be in part obviousand in part explained by reference to the accompanying specification anddrawings.

FIG. 1 of the drawings is a unit stereograph triangle referring tocrystallographic directions normal to the rolling plane whichschematically shows no (hkl) preferred orientation; i.e., the grainshave random orientations;

FIG. 2 is a unit stereograph triangle identical to that of FIG. 1 buttaken on tantalum processed according to the present invention; and

FIG. 3 is a unit stereograph triangle identical to that of FIG. 2 butmade from a different tantalum sample.

Broadly, the process of this invention comprises providing a cold rolledbody of tantalum foil of up to 6 mils thickness and subjecting this bodyto anneal at a temperature no lower than about 1950 C., and preferablyfrom 2100 C. to 2200 C., in a vacuum no greater than about 10- mm. ofHg.

As a metal, tantalum has a slow rate of work hardening and is notdiflicult to fabricate, standard methods and equipment being suitablefor its shaping. Most fabrication is effected at room temperature, dueat least in part 3,3 35,037 Patented Aug. 8, 1 967.

to tantalum, reactivity with the common gases when heated. Foilproduction is, of course, accomplished by cold rolling, the reduction insize per rolling stage generally ranging from 30 to 40 percent.Intermediate anneals are often used between rolling stages, theannealing temperatures being on the order of 1300 C. to 1400" C. for atleast 1 hour.

Referring to FIG. 1 of the drawings, circles 10 representcrystallographic axes normal to the rolling plane or plane of tantalumfoil having no preferred orientation. The absence of preferredorientation is indicated by the fact that circles 10 are evenlydistributed in the unit triangle. Such is not the case in connectionwith tantalum produced by this process, as clearly evidenced by FIGS. 2and 3 of the drawings where the material is indicated to have a (110)orientation.

Samples of tantalum foil were produced from two different sources oftantalum to produce (110) grain oriented foils of tantalum. The first ofthese was initially in the form of a V8 inch diameter tantalum rodinitially 99.79 percent pure which was then refined further in fourpasses of zone melting. The rod was cold rolled to 0.0065 inch thicknessand then annealed one hour at 1300 C. in a vacuum to 10'- mm. of Hg.This annealed strip was then cold rolled to .003 inch thickness andfinally annealed for 2 hours at 2100" C. in a vacuum of 10' mm. of Hg.The large grains in the annealed foil specimen were then analyzed bymeans of X-ray diffraction patterns to determine whether or not grainorientation had been achieved. FIG. 2 of the drawings was obtained byplotting the results from the diffraction pattern studies, and it isclear that a (110) preferred orientation was in fact obtained sincepoles of (110) planes of most of the grains studied were within 5degrees of an axis passing perpendicularly to the plane of the foil.

The second specimen was obtained using arc-melted tantalum of about thesame purity as the zone-melted tantalum. This material which hadpreviously been rolled to 0.010 inch thickness was annealed for 1 hourat 1400 C. in a vacuum to 10- mm. of Hg, then cold rolled to .003 inchthickness and finally annealed for 3 hours at 2200 C. The grainorientation of this specimen was studied in the same manner as that usedin connection with the zone-melted tantalum and the stereograph triangleshown in FIG. 3 of the drawings was obtained. Here, once again, it isevident that a strong (110) preferred orientation had been obtained. 1

It is felt that in order to obtain the orientation, the tantalummaterial must be of high purity, i.e. at least 99 /2 percent pure, sothat when subjected to the high temperature vacuum anneal, the grainboundaries within the material are free to migrate. It is felt that the(110) plane is one of lowest intrinsic energy so that given propergrowth conditions and enough such (110) oriented grains, their growthwill consume all other grains of different (hkl) orientation. The originof such selective growth might best be described as an example of an(hkl) surface energy phenomenon. It will be understood, however, thatthe preceding is oifered by way of explanation and is not intended to belimiting to the invention, since other phenomena might prevail.

Although the present invention has been described in connection withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

" 1 The'pr ocess for producing tantalum foil having the References Cited(110) crystallographic orientation comprising, provid- UNITEDSTATESPATENTS ing cold-rolled tantalum foil of up to 6 mils thickness and ofnot less than about 99.5 percent purity, and angvelss nealing thetantalum foil in a vacuum no higher than 5 3166414 1/1965 Femme {'175*174 10- mm. of Hg at a temperature no lower than about 3203793 8/1965g e a 1950 C. for a time sufficient to recrystallize the foil an anddevelop the (110) crystallographic orientation.

2. A process as described in claim 1 wherein the foil DAVID RECK"PmaryExammer' is annealed at temperatures of from about 2000- C. to 10 DEAN,Assistant Examiner- 2300" C. for'not less than about 1 hour.

1. THE PROCESS FOR PRODUCING TANTALUM FOIL HAVING THE (110)CRYSTALLOGRAPHIC ORIENTATION COMPRISING, PROVIDING COLD-ROLLED TANTALUMFOIL OF UP TO 6 MILS THICKNESS AND OF NOT LESS THAN ABOUT 99.5 PERCENTPURITY, AND ANNEALING THE TANTALUM FOIL IN A VACUUM NO HIGHER THAN10**-6 MM. OF HG AT A TEMPERATURE NO LOWER THAN ABOUT 1950*C. FOR A TIMESUFFICIENT TO RECRYSTALLIZE THE FOIL AND DEVELOP THE (110)CRYSTALLOGRAPHIC ORIENTATION.